Fabrication of soft contact lens and composition therefor

ABSTRACT

A contact lens composition, having improved dimensional stability, suitable for use in the preparation of a corrective hydrated lens or an eye bandage, comprises polymerized random graft copolymers containing (in the dry state) from 67.2% to 79.3% HEMA; from 14.25% to 35% PVP; from 0.1% to 4.04% EDMA; from 0.1% to 2.5% MA; from 0.1% to 5.0% water; from 0 to 4 ppm HQ inhibitor; and from 50 to 250 ppm MEHQ inhibitor. The contact lens composition contains from 45% to 65% water after hydration. An improved method for shaping, polymerizing, and cutting and hydrating the contact lens composition is provided. The polymerization is conducted using only a low temperature initiator, by de-gassing prior to polymerization to remove substantially all oxygen, and by conducting the initial polymerization reaction at from 23° C to 30° C for 16 to 30 hours, during which the heat of the reaction is continuously absorbed to control the reaction exotherm. A closed, rod-shaped casting mold, lined with a polyester film material, is used and the polymeric rod obtained may be cut, machined and polished to obtain the contact lens composition. Thereafter, hydration is conducted in a buffered, 0.85% to 0.9% saline solution, at a pH of 7.0 to 7.1.

This is a division of application Ser. No. 526,022 filed Nov. 21, 1974,now U.S. Pat. No. 3,978,164, granted Aug. 31, 1976.

BACKGROUND OF THE INVENTION

The present invention relates to a dimensionally stable contact lenscomposition obtained by carefully controlling the formulationingredients, the polymerization processes, and molding techniques. Thepolymerized product obtained is subjected to cutting and polishingoperations, followed by hydration in a buffered saline solution, toprovide a stable contact lens composition having uniform physicalproperties.

2. Description of the Prior Art

Contact lens compositions prepared from polymeric materials are known:

U.S. Pat. No. 3,220,960 discloses hydrophilic copolymers of a monoesterof acrylic or methacrylic acid with a diester of acrylic or methacrylicacid having desirable optical and physiological properties. U.S. Pat.Nos. 3,408,429 and 3,496,524 disclose various aspects of contrifugalcasting and manufacturing soft contact lenses using the hydrophiliccopolymers described in U.S. Pat. No. 3,220,960.

U.S. Pat. No. 3,700,761 discloses compositions comprising of HEMA(hydroxy-ethyl methacrylate) or HPMA (hydroxy-propyl methacrylate) withPVP in proportions of 20-45%, with up to 1% MA and about 0.2% EDMA. Atwo stage polymerization process, followed by radiation curing, cuttingand hydration provides the finished contact lens.

U.S. Pat. No. 3,816,571, 3,822,196, and 3,829,229 disclose variousaspects of the fabrication procedures generally described inaforementioned U.S. Pat. No. 3,700,761.

U.S. Pat. No. 3,807,398 discloses improvements in the U.S. Pat. No.3,700,761 by careful regulation of ingredient concentrations and theaddition of certain polymerization inhibitors to stabilize the HEMA. Inaddition, the polymeric composition is cast in a curved break away moldwhich eliminates the need for grinding the cast surface of the lens.

SUMMARY OF THE INVENTION

A dimensionally stable contact lens composition suitable for use inpreparing a corrective hydrated lens or an eye bandage is obtained byutilizing from 0 to 4 ppm of HQ polymerization inhibitor and from 50 to250 ppm of MEHQ polymerization inhibitor, based on the amount of HEMAmonomer, in the polymerization mix; initiating the polymerization with alow temperature, free radical initiator; de-gassing the polymerizationmix prior to polymerization to remove substantially all oxygen; andconducting the initial polymerization reaction from 23° C. to 30° C. for16 to 36 hours during which time the reaction exotherm is carefullycontrolled. A closed casting mold, lined with a polyester film material,is utilized for polymerizing and shaping the composition. The polymericcomposition obtained may be cut, machined and polished to obtain acontact lens composition which is then hydrated in a buffered salinesolution maintained at a pH of 7.0 to 7.1.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side exploded view showing the mold parts;

FIG. 2 is an end view of the tube portion of the mold with a polymerliner positioned therein;

FIG. 3 is a perspective front view of the flat polyester sheet; and

FIG. 4 is a perspective front view showing the coiled polyester sheet.

The mold of this invention is particularly suitable for polymerizingmaterials in the shape of a rod or the like which is to be latermachined into articles such as contact lenses. Examples of materials forwhich the mold is particularly suited for use include polymers andmixtures of polymers of methyl methacrylate, hydroxyethylmethacrylateand polycarbonates such as poly(diethylene glycol bis-allyl carbonate)or CR39 (Trademark of PPG Industries, Pittsburgh, Pa.). The mold of thepresent invention may be used for casting rods of any polymer of aprepolymer whose polymerization is inhibited by the presence of oxygen.One advantage of the mold of the present invention is the ease ofremoving the polymer rod from the mold after polymerization. Anotheradvantage of the mold, when used according to the present invention, isthe strain-free, dimensionally-stable characteristics of the polymerproduct. These characteristics are especially important if the ultimateproducts are contact lenses.

DESCRIPTION OF THE PARTICULAR EMBODIMENTS

It has now been found that a stable contact lens composition, suitablefor precision cutting and shaping, followed by hydration in a bufferedsaline solution, can be obtained by carefully controlling theformulation ingredients, the polymerization processes and the moldingtechniques. The stable contact lens product obtained has been found tobe predictable and reproducible both in the wet and dry states.Additionally, the improved contact lens composition remains stableindefinitely without change in critical dimensions.

Specifically, it has been found that while a polymerization inhibitor isneeded, the hydroquinone inhibitor adversely affects the HEMApolymerization as well as the polyHEMA/PVP graft polymerizationreaction. Therefore, no more than 4 ppm, based on the amount of HEMA, ofhydroquinone should be present for optimum polymer formation. The methylether of hydroquinone, which does not exhibit the disadvantages ofhydroquinone, has been found to be effective at levels of 50 to 250 ppm,based on the amount of HEMA, for inhibiting polymerization prior to theaddition of the polymerization initiator.

Additionally, it has been found that only a low temperature free radicalpolymerization initiator is necessary for complete polymerization. Priorart processes required an additional medium temperature free radicalpolymerization initiators. According to the teachings of this invention,initiators such as acetyl peroxide, disecondary-butyl peroxydicarbonate,cyclohexanone peroxide and the like, which operate between 23° C to 80°C., are suitable to initiate polymerization, and no additionalinitiators are necessary. Among these, disecondary-butylperoxydicarbonate, used at a level of 0.02% by weight, based on theweight of the polymerization mix is preferred.

The following formulation is suitable for preparing the dimensionallystable contact lens composition of this invention:

    ______________________________________                                        Ingredients  Amount                                                           ______________________________________                                        HEMA         67.2%          to 79.3%                                          PVP          14.25%         to 35%                                            MA           0.1%           to 2.5%                                           EDMA         0.1%           to 4.04%                                          Initiator    0.005%         to 0.2%                                           Water        0.1%           to 5.0%                                           HQ            0 to 4.0 ppm parts of HEMA                                      MEHQ         50 to 250 ppm parts of HEMA                                      ______________________________________                                    

A preferred formulation, corrected for impurities, for preparing theimproved contact lens composition of this invention contains:

    ______________________________________                                        Ingredients    Amount                                                         ______________________________________                                        HEMA           75.7%                                                          PVP            19.4%                                                          MA             1.2%                                                           EDMA           0.54%                                                          Initiator      0.02%                                                          Water          3%                                                             HQ             less than 4 ppm parts HEMA                                     MEHQ           100-200 ppm parts HEMA                                         ______________________________________                                    

Water in the above formulations is not added as such: it is inherentlypresent in ingredients such as HEMA and PVP, together with moistureabsorbed from the atmosphere prior to and subsequent to mixing andfabricating operations.

The above formulation is polymerized according to the improved processof this invention. Specifically, the formulation is de-gassed prior topolymerization to remove substantially all oxygen. Vacuum de-gassing,using known techniques, has been found effective when performed on thepolymerization mix in the casting mold, just prior to polymerization.Preferably, the de-gassing operation is performed twice: once on thepolymerization mix, after the addition of the polymerization initiatorand again after the polymerization mix has been poured into the castingmold. After the second de-gassing, the mold is back-flushed with aninert gas such as dry nitrogen, and the casting mold is immediatelyclosed to prevent further contamination with atmospheric oxygen. Thepresence of oxygen and the hydroquinone polymerization inhibitor havebeen found to seriously inhibit both the HEMA polymerization and thepolyHEMA/PVP graft polymerization reactions. Therefore, by removingsubstantially all oxygen, including atmospheric oxygen, before andduring polymerization, and by limiting the amount of hydroquinone toless than four parts per million, based on the amount of HEMA, bulkpolymerization of HEMA will take place at low temperatures.

High exothermic activity during the low temperature polymerizationreaction has also been found to adversely affect the physicalcharacteristics and uniform properties of the polymer formed. Thus,while the polymerization reaction may be conducted at temperatures offrom 23° C. to 30° C., with adjustments in amounts of polymerizationinitiator, the temperature selected is controlled so that the reactionexotherm does not vary more than about 4° C., preferably 2° C. duringthe 16-36 hour low temperature polymerization period. This may beaccomplished by placing the closed casting mold containing the de-gassedpolymerization mix in a medium which will continuously absorb the heatof the polymerization reaction. Preferably, this low temperaturepolymerization reaction is conducted by placing the mold in a constanttemperature water bath maintained at 25° ± 0.1° C. for about 24 hours.The closed mold is positioned in the bath so as to prevent contaminationof the polymerization mix with water from the bath. Most of the HEMA ispolymerized during this stage.

Next, the mold is removed from the heat absorbing medium and heated to atemperature of from about 40° C. to about 80° C. for from two to sixhours in order to complete polymerization of any remaining HEMA monomer.Preferably, the closed mold is placed in an oven maintained at atemperature of about 70° C. for approximately two hours.

The completion of the graft polymerization of the PVP and the HEMApolymer is achieved by heating the polymerized HEMA and the PVP to atemperature from about 105° to 125° C. for from 24 to 36 hours. Thisreaction can be performed after the reactants have been removed from themold since the polymerized HEMA is in the solid state. Preferably, aswill be explained below, the reactants, encased in a polyester filmmaterial, are heated in an oven maintained at 110° C. for 24 hours.

Substantially all bulk and graft polymerization reactions are completedafter the above-mentioned heat treatment. An additional polymerizationinitiator operable at medium to high temperatures is not required forcuring. This is particularly advantageous since the benzoyl peroxideinitiator used to complete HEMA polymerization in prior processes hasbeen found to react with hydroquinone present in the polymerization mixto form benzoquinone, which exerts a much more powerful inhibitingeffect than HQ on the polymerization reactions taking place. Using theimproved formulation and polymerization process of this invention, nofurther initiator or post-curing steps are necessary. A cured polymericproduct having substantially uniform physical properties is obtainedwithout irradiation or treatment with hydrogen peroxide previouslyrequired to achieve complete cure.

The casting and molding methods used to prepare the contact lenscompositions have also been found to be critical. Referring to FIG. 1,an oxygen absorbent tube 1 has an end plug 2 and a cap 3 as a convenientclosure for one end of tube 1. A split ring 4 or similar type device isuseful to provide a support for the mold during suspension in atemperature-controlled fluid bath (not shown). FIG. 2 is an end view oftube 1 having a polyester film coil 5 lining the interior surface oftube 1. FIG. 3 shows a sheet 6 of the polyester film which may be coiledas shown in FIG. 4 prior to insertion in tube 1. Mold materials, whichare critical to the practice of the present invention, include an oxygenabsorbent material for tube 1 and an oxygen non-absorbent polyester filmas the lining 5 for the tube 1. The oxygen absorbent properties ofpolytetrafluoroethylene are well known and tubes of the type illustratedare commercially available made of Teflon (Trademark of E. I. duPont deNemours, Wilmington, Delaware). A suitable oxygen non-absorbent materialfor the lining is exemplified by polyester films, especially thepolyethylene terephthalate films marketed commercially as Mylar (aTrademark of E. I. duPont de Nemours, Wilmington, Delaware). After themold is filled with the prepolymer mix, the other end of tube 1 isclosed with plug 7. Preferrably plugs 2 and 7 are also made of an oxygennon-absorbent polyester material.

The oxygen absorbent and non-absorbent properties of the mold and liningare particularly critical to the practice of the invention. Theseproperties enables one to conduct the polymerization in the mold andremove the rod cast by polymerizaton therein without difficulty ordamage to the rod or to the mold. The oxygen absorbent property of themold tube prevents polymerization of the mix in contact therewith.Therefore, any prepolymer mix which leaks between layers of the coil oraround the ends of the coil and contacts tube 1 does not harden and/orbond either the rod and/or the polyester lining to tube 1. Uponcompletion of the casting step it is therefore easy to remove the rodwith the polyester film from the tube and subsequently unwind the coilfrom around the rod. The mold size and shape is not critical and may bevaried to accommodate available lens fabricating equipment. Lens"buttons" or "rods" of polymerized lens stock from which the individuallens can be cut may be obtained according to the molding process of thisinvention. In a preferred embodiment, tubular, polytetrafluoroethylenecasting molds of from 330-380 millimeters in length and having insidediameter of approximately 15.8 millimeters are used. This open-endedrod-shaped mold is lined with the polyester film material and closedwith top and bottom cap plugs at various stages during the moldingoperation. For example, after the polyester lining is in place, thebottom cap plug is affixed and the polymerization mix is added to themold; the polymerization mix is de-gassed, the system is back-flushedwith an inert gas, and the top cap plug is affixed immediately toprevent contamination with atmospheric oxygen. The polymerizationreaction is then conducted as described above. Removal of thepolymerized mass from the mold is facilitated by the presence of thepolyester film lining: the top and bottom cap plugs are removed, and thepolyester encased self-supporting polymerized mass can be removed byexerting a small amount of pressure on either end of the polymerizedmass within the mold. The mold may be re-used after a relatively simplecleaning operation. The self-supported polymerized mass within thepolyester film lining is heated at 110° C. as described above, tocomplete the graft polymerization reaction. The polyester film is thenremoved from the polymerized rod and discarded. The cured rod stock thusobtained can be cut and machined easily on existing contact lensequipment.

Techniques for cutting, machining, and polishing the contact lenscomposition prepared as described above are known in the art and aregenerally described by Mullen in U.S. Pat. No. 2,330,837, Feinbloom inU.S. Pat. No. 3,227,507, O'Driscoll et al. in U.S. Pat. No. 3,700,761and by Gruzca in U.S. Pat. No. 3,807,398. Aforementioned U.S. Pat. Nos.2,300,837, 3,227,507, 3,700,761 and 3,807,398 are incorporated herein byreference.

Individual contact lens compositions which have been cut, machined andpolished are hydrated in a buffered saline solution according to theprocess of this invention. It has been found that the pH of thehydration operation is critical and must be conducted at a pH of fromabout 7.0 to about 7.1. A physiological saline solution (0.85% to 0.9%sodium chloride) is buffered with an ophthamalogically buffer which willnot interfere with the optical qualities of the lens, to maintain the pHof from about 7.0 to about 7.1. Suitably, a monobasic sodiumphosphate/dibasic sodium phosphate buffer system is used for thispurpose.

Hydration is conducted in the above-described solution for a timesufficient to remove water-soluble extractables from the lenscomposition and establish a constant set of hydration parameters. It hasbeen found that the extractable levels as low as from 2% to 3% areobtained using the improved formulating, polymerizing and moldingprocesses of this invention. Thus, hydration time is greatly reduced.Significantly, it has been found that by using a 0.9% saline solution,buffered to maintain a pH of from about 7.0 to 7.1, the equilibration ofthe contact lens composition of this invention is achieved in from fiveto seven days. In order to insure complete equilibration, the hydrationprocess is preferably conducted for seven days. Equilibrating andhydrating processes used for prior art contact lens compositionsfrequently required a thirty day treatment period and numerousprocessing steps. By reducing the extractable levels in the polymericcomposition and using the one-step hydration process of this invention,an improved hydrated contact lens composition is obtained in a muchshorter period of time.

The hydrated contact lens compositions of this invention are suitablefor use in correcting near-sightedness, far-sightedness and minorastigmatism. The contact lens composition of this invention may also beadapted for use as an eye bandage for dispensing medication.

As described by O'Driscoll et al. in U.S. Pat. No. 3,700,761 and byGruzca in U.S. Pat. No. 3,807,398, water soluble medicaments can beadded to the contact lens by dissolving the medicaments in water andadding 5% of the aqueous solution of the medicament to the HEMA monomerprior to polymerization. Especially useful eye bandages are prepared inthis manner. The additional water in the polymerization mix makes thelens, after hydration, spongy, and the medicament present is releasedwith greater facility. Opthalmologically acceptable medicaments suitablefor this purpose are known and include corticosteroids, sulfonamides,disinfectants, antiseptics, penicillin, pilocarpine, belladonna,dibenzyline, hydergine, methacholine, carbachol, bethanechol and thelike.

In preparing the contact lens composition of this invention for use aseye bandages to release medicament, formulating, polymerizing, moldingand hydrating procedures may be varied within the above-mentionedcritical limits to accommodate properties of the medicament being used.For example, heating temperature and times may be lowered to avoid drugdecomposition. Additionally, drug dosages for the eye which require agreat degree of precision can be administered using the shaped eyebandage of this invention. In such instances, it may be preferable toset up a fountain solution of the drug in saline and drop this solutiononto the lens to deliver the dosage to the eye at a constant rate for acontrolled time.

In all of the aforementioned descriptions of the contact lenscompositions of this invention, the abbreviations used are defined asfollows:

HEMA - Hydroxyethyl Methacrylate

PVP - Poly-N-Vinyl Pyrrolidone

EDMA - Ethylene Glycol Dimethacrylate

MA - Methacrylic Acid

ppm - parts per million

HQ - Hydroquinone

MEHQ - Methyl Ether of Hydroquinone

Thus, according to the teachings of this invention there is obtained adimensionally stable contact lens composition suitable for use inpreparing a corrective hydrated lens or an eye bandage having uniformphysical properties in both the wet and dry states, together with therequired strength and toughness for such lens compositions. The improvedcontact lens composition of this invention is obtained by criticallyselecting formulation ingredients and carefully controllingpolymerization, molding and hydration processes.

In order to further illustrate this invention, the following examplesare provided:

EXAMPLE I Preparation of the Contact Lens Composition

665 Grams of HEMA, 170 grams of PVP, 10.2 grams of MA, 4.76 grams ofEDMA, 0.001 grams of HQ, and .0065 grams of MEHQ are placed in a glasscontainer which is closed with a Mylar lined cap. The bottle is shakenfor 10 to 15 seconds to break up the PVP and then placed on a laboratoryrolling mill for 11/2 to 2 hours to complete solvation of the PVP. Thepremix is removed from the glass container and pressure filtered througha Seitz or pall filtering apparatus with 7 to 30 micron filer paper. Thefiltered premix is refrigerated at 0° to 4° C.

0.17 Grams of disecondary-butyl peroxydicarbonate is added to the coldpremix and the initiator-containing mix is then vigorously shaken forten seconds, after which it is placed on a rolling mill for 30 minutes.The mix is then transferred into a suitable container and de-gassed for10 to 15 minutes at 1-50 mm of reduced pressure. When releasing thevacuum, the chamber is back-flushed with dry nitrogen.

A 5 mil Mylar sheeting is cut into strips and rolled onto a steel rodhaving a diameter of 12.7 mm and inserted into a Teflon casting tube 330millimeters long having an inside diameter of 15.8 millimeters and anoutside diameter of 19.0 millimeters. The liner is wiped clean of grossparticulate matter prior to insertion and the tube is blown out with drynitrogen from both ends following the insertion of the Mylar liner. Thebottom cap plug, which has been soaked in acetone for 24 hours prior touse, is affixed to the casting tube and the polymerization mix is pouredinto place. The casting tube containing the polymerization mix isde-gassed in a vacuum chamber for 20-40 minutes at 1-50 mm of reducedpressure. The system is back-flushed with dry nitrogen as the vacuum isreleased. The casting tube is removed from the vacuum chamber and thetop cap plug is affixed tightly immediately.

The closed casting tube is placed in a rack in a constant temperaturewater bath maintained at 25° C. ± 0.1° C. for 20 hours, with the closedcasting tube positioned so as to prevent contamination of thepolymerization mix with water from the water baths. The casting tube isthen transferred to an air circulating oven set at 70° C. ± 1° C. fortwo hours. The casting tube is then taken out of the oven and thecasting is taken out of the tube by removing the cap plugs and thenusing a steel or aluminum poking rod. The casting, encased in the Mylarlining, slides easily out of the tube. The liner is not removed and theencased casting is placed in an air circulating oven, set at 110° C. ±1° C. for 24 hours.

The Mylar liner is then removed; the cast rods are visually inspectedand stored in sealed glass or Mylar containers until needed. The emptycasting tubes are cleaned by soaking in water, rinsing and allowing todry before reuse.

The hard polymerized cast rods are subjected to known machining, cuttingand polishing techniques as described in U.S. Pat. Nos. 2,330,837,3,227,507, 3,700,761, and 3,807,398, to form individual lenscompositions.

EXAMPLE II Hydration of the Contact Lens Composition

A buffered physiological saline solution is prepared from 8.50 grams ofreagent grade sodium chloride, 0.2545 grams monobasic sodium phosphate,0.7006 grams dibasic sodium phosphate and distilled water, added to1,000 ml. This buffered solution will maintain a pH of about 7.1 Theindividual contact lens compositions of Example I are hydrated in thissolution at 22° C. to 25° C. for seven days. Thereafter, the hydratedlens compositions are stored indefinitely in saline solution.

We claim:
 1. A shaped eye bandage for medicament delivery which can becut and shaped in the dry form and thereafter hydrated in a bufferedsaline solution to form a bandage having from about 45% to 65% waterafter hydration, said bandage in dry form consisting essentially ofpolymerized random graft copolymers containing from about 67.2% to about79.3% HEMA; from about 14.25% to about 35% PVP; from about 0.1% to about4.04% EDMA; from about 0.1% to about 2.5% MA; from about 0.1% to about5.0% water; from about 0 to about 4 ppm, based on the weight of HEMAmonomer, of HQ inhibitor; and from about 50 to 250 ppm, based on theweight of HEMA monomer, of MEHQ inhibitor; said composition having beenpolymerized in a three-stage polymerization process comprising:A. afirst stage, using only a low temperature polymerization initiator, withsubstantially all the oxygen having been removed from the polymerizationmix prior to and during the polymerization reaction; the initialpolymerization reaction being conducted at from 23° C. to about 30° C.,for from about 16 to about 36 hours, with the heat of the reactioncontinuously absorbed to control the reaction exotherm; followed by B. Asecond stage, wherein the product of (A) is heated to a temperature offrom about 40° C. to about 80° C. in the absence of any additionalpolymerization initiator, without control of the reaction exotherm, forfrom 2 to 6 hours; followed by C. A third stage polymerization, whereinthe product of (B) is heated to a temperature of from about 105° C. toabout 125° C., in the absence of any additional polymerizationinitiator, for from 24 to 36 hours;said bandage containing anopthalmologically acceptable medicament incorporated therein.
 2. Ashaped eye bandage according to claim 1 wherein about 75.7% HEMA, about19.4% PVP, about 0.54% EDMA, about 1.2% MA. about 3.0% water; from 0 to4 ppm, based on the weight of the HEMA monomer, of HQ inhibitor, andfrom about 100 to 200 ppm, based on the weight of HEMA monomer, of MEHQinhibitor are present in the substantially dry form of the contact lenscomposition, said dry form of the contact lens composition being cut andshaped and thereafter hydrated to contain from about 45% to about 65%water, said hydration being conducted in a 0.85% to 0.9% saline solutionin combination with a fountain solution of an opthalmologicallyacceptable medicament, dissoolved in saline solution, said salinesolution being buffered to maintain a pH of from about 7.0 to 7.1.
 3. Ashaped eye bandage according to claim 1 containing an opthalmologicallyacceptable medicament incorporated in the dry form of the bandage.